This invention relates to drag reduction coatings and more particularly to coatings for reducing flow resistance and turbulent drag at the interface of the coating with the liquid and down stream of the coated area.
Prior art methods for reducing the drag encountered by a moving object such as a torpedo depend chiefly on carrying and ejecting a solution of long chain, high molecular weight, water soluble polymer into the turbulent boundary layer as the moving object speeds through the water. The technology advanced from carrying and ejecting the dissolved, highly concentrated solutions to making the solutions on board by mixing of the polymer with ingested sea water. The high viscosity of concentrated solutions limits the usable concentrations to those that can be pumped efficiently without unduly shearing the high molecular weight polymers. The prior art further advanced from the use of concentrated solutions to polymer slurries. The particle size of the polymers used in slurries must be very small for rapid solution. The fine powder must be suspended in a liquid in which it is insoluble and must not grow in particle size or agglomerate during storage. The liquid must not adversely affect the subsequent solubility of the suspended particles in water. The ejection of drag reduction material into turbulent boundary layers required pumping, mixing and metering devices; on board storage of materials (i.e., solutions or slurries etc); to be ejected and perforation of the torpedo hull. These systems inherently require space that could be used for more desirable purposes. Furthermore, they add weight to the existing system and contribute to the generation of noise. It is thus desirable to have systems which can be designed to use drag reduction material coatings which can be applied to the moving body with ease. Such coatings are one shot systems and they can be easily applied to a moving object-water interface before the object is put in the water or in motion. Unlike solutions or slurries, coatings do not require equipments and devices for dissolving and ejecting metered quantities of the materials into the boundary layer. The rate of solution is integrated into the coating composition, formulation and fabrication. The concentration in the boundary layer is also controlled by selected location, size and geometry of the coating applied to the surface of the moving body. The rate of solution accommodatingly increases with the speed of the moving body. The flow pattern around the moving body introduces and distributes the dissolved polymer into the boundary layer. However, the usefulness, areas of application and efficiency of drag reduction coatings developed to date are restricted because of the lack of high molecular weight polymer coatings and because of the high concentration, high viscosity and drag resistance at the coating-water interface of very high molecular weight water soluble polymers. The systems of prior art therefore use less efficient, relatively low molecular weight, rapidly soluble drag reduction polymers in an attempt to decrease the viscosity at the coating-water interface. They use coating formulations which are rapidly soluble in water at the stagnation and low velocity laminar flow areas of the streamlined nose section of a moving body such as a torpedo and/or limit the coated areas exposed to the turbulent flowing liquids. The solution is then swept into the turbulent boundary layer from which it proceeds downstream over the uncoated areas of the moving body. The use of low molecular weight rapidly soluble polymers limits the time a coated vehicle can be immersed in water before it is fired as well as it limits the operational life-time of such coatings. Immersion of the body prior to firing creates high viscosity areas adjacent to these coatings. Furthermore, it dissolves and swells the coating in stagnant water and increases surface roughness and rapid ablation when the moving body travels through the water at high velocities. Thus, in order to improve the present drag reduction systems using slurries or coatings, it is desirable to use high molecular weight polymers coatings which do not produce high concentration and high viscosity at the coating-water interface.